1. Field of the Invention
The present invention relates to an anti-pinch sensor, in particular for detecting an obstacle in the path of an actuator of a motor vehicle, having a first measurement electrode and having a second electrically isolated measurement electrode spaced a distance from the first measurement electrode, wherein the first measurement electrode is designed to generate an external electric field with respect to the second measurement electrode. The invention further concerns a sensor device having such an anti-pinch sensor, wherein a measurement electronics unit is provided for evaluating the measured capacitance between the measurement electrodes.
2. Description of the Background Art
An anti-pinch sensor of the aforementioned type uses the capacitive measurement principle to detect an obstacle. To this end, an external electric field is produced by means of the two measurement electrodes. If a dielectric enters this electric field, the capacitance of the capacitor formed by the measurement electrodes changes. In this way, it is theoretically possible to detect an obstacle in the path of an actuator of a motor vehicle as long as its relative dielectric constant ∈r is different from the relative dielectric constant of air. A change in the capacitance is also produced in the event that the intruding obstacle is connected to ground, since the charge distribution on the measurement electrodes is changed thereby.
The obstacle in the path of an actuator is detected without physical contact with the anti-pinch sensor. In other words, what is known as a contactless sensor is involved here. If a capacitance change is detected, countermeasures, as for example stopping or reversing the drive, can be initiated in time, before actual pinching of the obstacle occurs. In the case of actuators in a motor vehicle, this can involve an electrically operated window, an electrically operated sliding door, or an electrically operated tailgate, for example. An anti-pinch sensor based on the capacitive measurement principle can also be employed for detecting obstacles in the case of an electrically operated seat.
An anti-pinch sensor of the aforementioned type is known from U.S. Pat. No. 6,972,575 B2, for example. There, a ground electrode is located between the two measurement electrodes, and effects a shielding on the direct connecting line between the measurement electrodes. In this way, the sensitivity of the anti-pinch sensor to obstacles in the external electric field is increased.
Also, an anti-pinch sensor of the aforementioned type is known from EP 1 455 044 A2, where, in addition to a capacitance sensing between the two measurement electrodes, electrical contact with them is detected in the event that an external force acts. For this dual function as both a contactless and tactile sensor, the measurement electrodes are embedded in an elastomeric material and are spaced apart from one another by a hollow chamber.
In addition, an anti-pinch sensor of the aforementioned type is described in EP 1 154 110 A2, in which the measurement electrodes creating the external electric field are designed to be movable relative to one another. This is accomplished, for example, by a hollow space or by means of an elastic material. Through detection of a change in the capacitance of the capacitor composed of the measurement electrodes, an obstacle is detected both in a contactless manner during its approach and by direct contact with the sensor, wherein the distance of the measurement electrodes from one another changes.